1. Field of the Invention
The present invention relates generally to a cross-gain modulation type optical wavelength converter, and more particularly to a cross-gain modulation type optical wavelength converter which is capable of preventing an extinction ratio from lowering and allowing an input dynamic range to widen.
2. Description of the Prior Art
As the amount of transmitted information has rapidly increased recently, demands on transmission capacities of communication networks have increased and the capacities of transmission systems tend to grow increasingly large due to the increasing demands. In this circumstance of various technologies of increasing a transmission velocity that have been proposed up to now, a Wavelength Division Multiplexing (WDM) transmission technology has been most actively studied in that communication can be carried out over a wide bandwidth provided by optical fiber by utilizing the optical wavelengths of various channels. An optical wavelength converter is an important element of a communication network using a WDM transmission apparatus, along with a semiconductor optical amplifier (SOA). Such an optical wavelength converter generally converts the wavelength of a signal using the cross-gain modulation (XGM), cross-phase modulation (XPM) or photo-electric-photo conversion of an optical signal and a probe light.
In general, wavelength converters are apparatuses that convert wavelengths of transmission signals without regard to transmission velocities and transmission methods. The wavelength converters serve to reduce wavelength blocking caused by wavelength contention in WDM communication networks and increase the flexibility and capacity of the networks for fixed wavelengths by reusing useful wavelengths. Additionally, the wavelength converters serve to allow networks to be managed while being distributed, and additionally serve to allow protection switching to be easily performed.
There are various embodiments of the optical wavelength converters, which may be classified into three types. The first is an XGM type using the XGM characteristics of an SOA. The second is an XPM type using the XPM characteristics of an SOA. The third is a Four Wave Mixing (FWM) type, which generates optical signals having new wavelengths and wavelength-convert the optical signals through FWM. The XGM type is most commonly used and simplest to implement.
Such a conventional XGM type optical wavelength converter is shown in FIG. 1. Referring to FIG. 1, a pump light 102 is an intensity-modulated input optical signal having a wavelength of xcexs. A probe light 106 is a Continuous Wave (CW) light having a wavelength xcexc, which is outputted from a CW light source 104. The pump light 102 and probe light 106 are simultaneously inputted to an SOA 101. The gain of the SOA 101 is modulated by the pump light 102. The probe light 106 is influenced by the gain modulation Of the SOA 101, so the probe light 106 is intensity-modulated in the same manner as the pump light 102. Since the SOA 101 outputs the pump light 102 and the wavelength-converted probe light at its output terminal, a Band-Pass Filter (BPF) 107 is employed to pass therethrough only a wavelength-converted light 103.
The conventional XGM type optical wavelength converter is problematic in that the extinction ratio of a wavelength-converted signal outputted from the SOA at the time of wavelength conversion is lowered due to the gain characteristics of the SOA, a bit stream is reversed and an input dynamic range is narrow.
Meanwhile, a method of reducing the gain recovery time of an SOA so as to wavelength-shift optical signals of a high bit rate in a wavelength-shifter using XGM characteristics in the SOA, is disclosed in U.S. Pat. No. 5,450,229 entitled xe2x80x9cOptical wavelength-shifter with reduced gain recovery timexe2x80x9d and issued to Jay M. Wiesenfeld. This patent provides a method of wavelength-shifting optical signals of a high bit rate by using characteristics of reducing gain recovery time of an amplifier by the intensity of a CW light and, thus, reducing rise time at the time of XGM of the CW light in a general optical wavelength-shifter, thereby wavelength-shifting the optical signals having a bit rate of 10 Gb/s or higher using the gain saturation characteristics of a SOA.
Furthermore, in xe2x80x9cTechnique for Suppression of Pattern Dependence in a Semiconductor Optical Amplifier Wavelength converterxe2x80x9d, IEEE Photonics Technology Letters, Vol. 9, No. 12, pp. 1583-1585, December 1997, a method of reducing pattern dependence in a wavelength converter using XGM characteristics in an SOA was proposed by D. Mahgereft eh et al., in 1997. Generally, wavelength conversion using XGM characteristics in an SOA is a reaction dependent on the pattern of the SOA, and has a limitation in a bit rate of data to be converted. Accordingly, in this method, the pattern dependence is solved by converting phase modulation to amplitude modulation at the time of data transition using a fiber grating filter.
In the conventional XGM type optical wavelength converter, the lowering of the extinction ratio of a wavelength-converted signal outputted from the SOA due to the gain characteristics of the SOA at the time of wavelength conversion and the narrowing of the input dynamic range cannot be fundamentally overcome.
The present invention provides an optical wavelength converter, which is provided with a high extinction ratio and a wide dynamic input range by allowing a probe light to be primarily influenced by cross-gain modulation of the first SOA caused by the first pump light and to be secondarily influence by cross-gain modulation of the second SOA caused by the second pump light, and by detecting the intensity of an optical signal inputted to the optical wavelength converter and automatically controlling the intensity of the probe light on the basis of the detected intensity of the input optical signal, respectively.
The foregoing and other objects of the present invention are achieved by providing a cross-gain modulation type optical wavelength converter converting the optical wavelength of a probe light outputted from a continuous wave light source by using an intensity-modulated pump light, comprising: an optical division unit for dividing the intensity-modulated pump light into first pump light and second pump light on a basis of a predetermined light intensity ratio; a first control unit for detecting an intensity of the first pump light and controlling an intensity of the probe light outputted from the continuous wave light source in proportion to the detected intensity of the first pump light; a first semiconductor optical amplifier for performing a cross-gain modulation by using the first pump light and first converting the wavelength of the probe light outputted from the continuous wave light source on a basis of the cross-gain modulation; a second control unit for controlling an intensity of the first wavelength-converted probe light outputted from the first semiconductor optical amplifier in proportion to an intensity of the second pump light; and a second semiconductor optical amplifier for performing a cross-gain modulation by using the second pump light and second converting the wavelength of the first wavelength-converted probe light intensity-controlled by the second control unit on a basis of the cross-gain modulation.
It is preferable that the optical wavelength converter further comprises an optical phase control unit for controlling a phase of the second pump light so as to be in phase with the first pump light.
Additionally, it is preferable that the first control unit comprises a first optical extraction means for extracting some of the first pump light, a first optical detection means for detecting the extracted first pump light and converting the extracted first pump light into an electrical signal, and a light source drive control means for controlling the continuous wave light source using the electrical signal and a preset electrical offset signal; and the second control unit comprises a second optical extraction means for extracting some of the second pump light, a second optical detection means for detecting the extracted second pump light and converting the extracted second pump light to an electrical signal, a second optical amplifier for amplifying an output signal of the second optical detection means by a certain amount, and a variable optical attenuator for controlling an attenuation intensity of the first wavelength-converted probe light outputted from the first semiconductor optical amplifier.
The present invention provides an XGM type optical wavelength converter having a high extinction ratio and a wide input dynamic range, which, in particular, prevents an extinction ratio from lowering and allows an input dynamic range to widen at the time of optical wavelength conversion using XGM characteristics in the SOA. The XGM type optical wavelength converter of the present invention allows a probe light to be one more time influenced by the cross-gain modulations of two SOAs caused by each pump light respectively, and allows the width of the modulation of the probe light to be further broadened, thereby improving the extinction ratio after wavelength conversion. Simultaneously, the XGM type optical wavelength converter of the present invention detects the intensity of an optical signal inputted to the optical wavelength converter and automatically controls the intensity of the probe light on the basis of the detected intensity of the input optical signal using the characteristics of performance of the XGM type optical wavelength converter on the basis of the intensity ratio of a pump light and a probe light inputted to each of the SOAs, thereby providing a wide input dynamic range. Accordingly, a low extinction ratio problem occurring in the conventional XGM type optical wavelength converter using a single SOA is overcome and, simultaneously, another problem that the performance of the optical wavelength converter varies very sensitively with the intensity of the input optical signal and that occurs in the conventional XGM type optical wavelength converter non-controlling the intensity of the probe light is overcome, so an operation bandwidth is significantly broadened with respect to the variations of the intensity of the optical signal inputted to the optical wavelength converter. Additionally, the Bit Error Rate (BER) characteristics of the optical wavelength converter of the present invention are improved compared to that of the conventional optical wavelength converter, and wavelength conversion can be performed with a constant extinction ratio being maintained with respect to the wide variations of the input optical signal.